1. Field of the Invention
The invention described herein relates to polyimide compositions that are useful as high refractive index materials. These compositions have good solubility (>10% w/w) in suitable solvent systems, high molecular weights, good film strengths, high transparencies in the visible and near-infrared region, and exceptional refractive indices.
2. Description of the Prior Art
High refractive index coatings increase the operating performance of many optoelectronic devices. For example, the efficiency of light-emitting diodes (LEDs) is improved by applying a layer of high refractive index material between the device and the encapsulating material.
Many organic polymer systems offer high optical transparency and ease of processing but seldom provide high refractive indices. The typical use of high refractive index organic polymers has been for ophthalmic lens manufacture. Among these polymers are bisphenol A polycarbonate with a refractive index of 1.58, polyphosphonates with refractive index values of 1.58 to 1.64, some novel acrylate and methacrylate polymers with reported refractive indices from 1.53 to 1.63, and sulfur-containing polymers with refractive indices ranging from 1.61 to 1.71.
A different approach to the formation of high refractive index film materials has been through the incorporation of inorganic compounds into polymer systems, resulting in hybrid materials. Various titanium dioxide (titania) hybrid materials have been reported in the literature, with refractive indices ranging from 1.505 to 1.867. However, formation of the inorganic component is accomplished by the sol-gel route, which usually necessitates the addition of water and the utilization of high temperatures (i.e., higher than 300° C.) for the proper formation of the titania nanoparticles. Other potential problems include the occurrence of microphase and macrophase separation, leading to optical loss and the inability to form relatively thick films.
Polyimides have been used extensively in microelectronics because they possess many desirable properties such as thermal stability, oxidative stability, high mechanical strength, and excellent solvent resistance. However, these same properties also give rise to problems such as difficulty in processing and manufacturing, as well as extreme insolubility in common organic solvents. Among the interests in polyimide use for microelectronics applications has been as interlayer dielectric materials. For this application, the desire has been to achieve lower dielectric constants in order to increase device efficiency by reducing signal distortion from capacitive coupling and crosstalk. Because dielectric constant and refractive index are exponentially related (the approximate relationship from the modified Maxwell equation: ε˜1.10 n2, where ε is the dielectric constant and n is the refractive index), the reduction in dielectric constant values results in small decreases in the refractive index. The reduction of polyimide dielectric constants has been accomplished by various synthetic approaches such as the use of semiaromatic, aliphatic, and fluorinated dianhydrides and diamines.
There is a need for high refractive index materials that also have good transparencies over a wide range of wavelengths, good solubilities in acceptable solvents, high molecular weights, and the ability to form films of at least 5 microns thick by spin coating or casting from solution.